Conventional Stirling Cycle Rotary Cooling Engines generally have a compressor and an expander connected to a crank mechanism driven by an electrical motor. The compressor, also known as a pressure wave generator. It is attached to the warm end of the expander and delivers acoustic power (compressor PV work) into the expander warm end inlet. Compressor PV work is the integration of the pressure-volume curve over one thermodynamic cycle or one complete revolution of the crank shaft. Compressor PV work has a unit of energy, and when derived over time, it is defined as acoustic power. The expander recovers this work at the cold end by causing the gas to expand and thus absorb heat from external power source such as an IR sensor. The gas expansion is achieved mechanically by placing the expander piston and compression piston at 90 deg mechanical phase to each other relative to the crank shaft. A working fluid, typically a noble gas, is compressed at the warm end and is expanded at the cold end. At the distal tip of the expander coldwell, when the expander piston is being pulled backward to iso-thermally expand the working gas, heat is absorbed from the load and very low temperatures are achieved due to efficient thermal isolation between the warm and cold end of the expander unit. Temperature can reach down to the cryogenic range, e.g., about 77° K. An infrared (IR) sensor, which needs to operate at such low temperatures, is attached to the coldwell to be cooled. A conventional Stirling engine is described in U.S. Pat. Nos. 7,555,908 and 7,587,896 and references cited therein, which are incorporated herein by reference in their entireties. Stirling engines are commonly used as cryogenic coolers to cool IR sensors for IR cameras and the like.
A conventional expander 1, illustrated in FIG. 1, generally consists of cold finger 2, which is a small diameter, thin-wall cylinder/tube, and a displacer unit 3 positioned in the cold finger. Displacer unit 3 comprises a canister tightly packed with metallic fine mesh, spheres or felt-like material, and moves within cold finger 2. The metallic fine mesh, spheres or felt-like material is also known as the regenerator matrix and is designed to exchange thermal energy with the working fluid. Displacer unit 3 is slip fit into cold finger 2 to provide precise linear reciprocating motion between the cold finger and the displacer. Working gas from the warm end enters expander 1 at the proximal end of displacer unit 3 at inlet 4. Since displacer unit 3 undergoes reciprocating motion, inlet 4 is static while inline with a moving slotted inlet machined into the displacer at the warm end clearance dynamic seal 5 and thus allows free flow into the regenerator regardless of its position. Reciprocating dynamic seals 5 prevent leakage of the working gas as it enters the moving displacer unit. Also, it prevents the cold gas present in the clearance between the displacer and the expander cylinder from escaping into to warm end during the expansion portion of the cycle. The working gas then enters the regenerator matrix to exchange thermal energy with the regenerator, and is pre-cooled. The working gas reaches the distal end of the displacer unit proximate to coldwell 6 ideally at the same temperature it left at the previous cycle right after the expansion. For an infrared-based system, an IR sensor is attached to coldwell 6 to be cooled.
The reciprocating motion of displacer unit/canister 3, more specifically the movement away from coldwell 6, isothermally expands the working gas causing it to cool down and absorb heat from the thermal load. Subsequently the expander piston/displacer moves toward the end cap and forces the working gas to flow back toward the warm end through the regenerator matrix to exchange thermal energy therewith, and is warmed. Hence, displacer unit 3 functions both as a displacer and regenerator. Displacer unit 3 also functions as piston and thus performs the expansion process in the thermodynamic cycle. The design of such an expander in which the displacer unit performs three different functions, i.e., displacer, regenerator and expansion piston, requires the system engineer to perform trade offs among various system requirements which can be often conflicting.
For example, the need to provide thermal barrier/insulation between the warm end and the cold end favors the cold finger 2 be long, thin and have a small diameter, since heat conduction along tube 2 would be minimized. On the other hand, the demand for miniaturization and rigidity of expander 1 favors the opposite. One major challenge when attempting to reduce expander length is the need to maintain a predetermined surface area for a given mass flow rate and cooling capacity by the regenerator matrix.
A regenerator used in a Stirling engine can be thought of as a one-way and a bidirectional heat exchanger in which thermal energy flows in and out of the matrix and to or from the working gas. The heat exchanging media, i.e., the regenerator matrix, is usually made of light felt-like mass of fine wire stacked in an insulated tube as shown in FIG. 1. The fine wire mesh is commonly obtained in a form of woven screen in a variety of wire sizes, weave structures, mesh density and materials. Other known types of regenerator matrices use spheres made of stainless steel, bronze, lead and erbium, among others. Common Stirling engine regenerator matrices usually have large thermal capacity, large surface area, low flow impedance, small void volume and large axial thermal resistance to achieve high regenerator effectiveness. Cooler performance is sensitive to regenerator effectiveness. A regenerator is considered to be “100% effective” when the temperature of the working fluid exiting the regenerator is equal to the temperature of working fluid entering it. When the temperature of the gas leaving the regenerator at the compressor end is colder than the entering gas, it indicates that not enough thermal energy was exchanged with the regenerator matrix. This causes the regenerator to be warmer than it could have been, thus reducing the pre-cooling of the incoming gas prior to it entering the expansion space. It is a challenge to minimize the length of the expander while maintaining efficient thermal exchange, i.e., adequate regenerator surface area, minimum pressure drop, large axial thermal resistance along the regenerator, large thermal capacitance and minimum weight.
The conventional expander assembly overall length LE shown in FIG. 1, is determined primarily by the regenerator length LR, while regenerator length LR is determined by expander 1's need for large matrix surface area, regenerator tube thermal resistance, regenerator matrix thermal contact resistance and shuttle losses consideration. Satisfying these design constraints has resulted in a relatively long expander assembly length LE and thus limits the ability to miniaturize the overall cryogenic cooler.
Hence, there remains a need for an improved cryogenic cooler that is further miniaturized and more specifically for a shorter, more compact expander.